Nowadays most hostels supply sheets but now and again you need (or maybe even WANT) your own. With your own travel sheet, you are assured a clean sheet (assuming you keep your sheet clean), it can help regulate heat and sweat, you always have a blankie of sorts with you, and it even covers a possibly not-so-desirable hostel pillow.
Travel sheets are synonymous with sleeping bag liners; liners that you slip into your bag to help with temperature, cleanliness, and I'm not sure what else. But for those of us using them for travel and not camping, we'll call them travel sheets.
As with any backpacking travel gear, high tech is better, lighter, stronger. Brands such as Sea to Summit, DreamSacks, and many others make all sorts of acceptable sleeping bag liners. Feel free to research those by weight, fabric, and size and shape (mummy vs. regular). But I'm not reviewing any of those "other" travel sheets. I have a specific requirement for my travel sheet that not many have ....

The Cocoon Coupler comes in silk, silk/cotton (my fave), and cotton.
I absolutely cannot sleep in a confined sleeping bag liner. Picture yourself trying to curl up in a ball in a mummy sleeve or trying to put your foot outside the sheet if you get hot. Nope, not gonna work. Unless you have a zipper.
You can zip it all the way up if you want to be secured inside your sheet, or you can open it part way or even unzip it entirely to make a fully flat sheet. If you are traveling as a twosome, you can even zip two liners together to make a big queen-sized sheet sack! Pretty romantic, eh?
There are very few brands that make them with a full length zipper (side and bottom), hence those are the only ones I will ever even consider purchasing. I personally own the Cocoon Coupler in a silk/cotton combo. Even though it costs more, it's a quality product. My Cocoon has gone through 10 years of heavy use and is still intact! I bought the poly-cotton one for my step-daughter. It takes up a bit more room but it's acceptable and comes in a cute pattern (because that matters LOL).
The zipper does add a little weight and bulk, but it's so worth it. Cotton also adds weight and bulk but is also the least expensive.
For me, the Cocoon silk/cotton combo is the perfect balance. It rolls up to about the size of a tall coffee mug and stuffs in it's own sack. Unfortunately it's hard to find online at the moment. My sheet is quite the little luxury item and I just love slipping into the cool silky fabric when everything else around me is grubby and backpacker'y.